I think at some level, it's just alchemy that we, as writers, can't explain when we write the characters. I don't set out to create the characters - they're not, to me, collections of quirks that I can put together. I discover the characters, instead. I usually go through a standard set of interview questions with the character in the beginning and ask the vital stuff: What's important to you? What do you love? Hate? Fear? .. and then I know where to start. But the characters just grow on their own, at a certain point. And start surprising me.